In general, when a smartphone is in a mute mode or a vibration mode, and the owner of the smartphone forgets where the smartphone is or the smartphone is lost, it is very difficult to find the smartphone in a short time since the smartphone does not generate any sound even the user makes a phone call to the smartphone.
A conventional method for finding the smartphone under the above condition is based on a Bluetooth technology. However, the conventional method requires an additional device in or with the smartphone, such as a BLE (Bluetooth Low Energy) tag, and the additional device can be costly for ordinary users. In addition, the conventional method can only be used in a one-way operation rather than a bi-directional operation. For example, the conventional method can only use a BLE tag to find the smartphone, but the conventional method is not capable of using the smartphone to find the BLE tag.